In machines employed in the production and/or checking of semiconductor components, for example, it is often necessary to accurately position a processing tool in relation to a workpiece. For instance, it may be necessary to position a workpiece in the form of a wafer underneath a processing tool in extremely precise manner A corresponding system is shown schematically in FIG. 1. The wafer, or workpiece WS, is situated on a first object O1 of the machine, which is a table. In addition, an object alignment mark M1 is provided on first object O1. It may be arranged, for example, as a circular depression in the table or as lithographically produced microstructure on a plate fixed in place on the table. Moreover, a workpiece alignment mark M2, which may be arranged as a locally restricted microstructure, for example, is provided on workpiece WS, or the wafer, mounted on first object O1.
A second object O2, i.e., a corresponding machine part, is provided in the machine such that it is able to move in at least one movement direction x in relation to first object O1. For example, the relative mobility between both objects O1, O2 may be ensured in that object O1, which is the table in the present example, is disposed in a manner that allows it move at least along movement direction x, whereas second object O2 is stationary.
A processing tool B and an alignment sensor W are provided on second object O2. Processing tool B is able to machine or inspect the workpiece during the production process.
Alignment sensor W is arranged as a microscope or camera equipped with an electronic image sensor, for example. Object alignment mark M1 as well as workpiece alignment mark M2 are detectable by alignment sensor W. That is to say, if the position of alignment sensor W along movement direction x matches the position of object alignment mark M1 or of workpiece alignment mark M2, then a corresponding alignment signal is able to be generated via alignment sensor W. The alignment signal may then be transmitted for further processing to a downstream control unit, which is not shown in FIG. 1.
Another alignment signal is furthermore able to be generated via an additional sensor system (not shown in FIG. 1) if the position of processing tool B along movement direction x matches the position of object alignment mark M1. This may be done such that, for example, a microscope or a camera having an electronic image sensor is disposed on the side of object alignment mark M1 as well. If processing tool B emits radiation, corresponding radiation detectors, which detect the radiation reflected, transmitted or dispersed by object alignment mark M1, may be used as sensor system. Conversely, it is also possible that object alignment mark M1 transmits radiation which is able to be detected by radiation detectors in processing tool B. To generate such an alignment signal, there is furthermore the alternative of detecting a mechanical contact between processing tool B and object alignment mark M1 via an electrical contact current or suitable force sensors.
An optically scannable measuring standard S, for example, which extends along the at least one movement direction x, is provided on second object O2 so as to determine the relative position between first object O1 and second object O2. This measuring standard S may be arranged as a conventional reflected light diffraction grating, for example. To scan measuring standard S, a scanning unit E of an optical position measuring device is disposed on the side of first object O1. Scanning unit E may include, for example, a light source, a plurality of optical elements and a detector system so as to generate, with the aid of the detector system and in, e.g., a conventional manner, using an interferential scanning principle, exceedingly precise position signals regarding the relative position of the first and second object O1, O2, respectively, for the downstream control unit.
During the processing of the workpiece by processing tool B, it will now be necessary to determine the position of processing tool B relative to workpiece alignment mark M2 in extremely precise manner. Since no position signals for the direct alignment of processing tool B in relation to workpiece alignment mark M2 are able to be generated, this determination must take the form of a sequence of individual calibration steps.
For example, referencing of processing tool B in relation to first object O1 with the aid of alignment mark M1 is required. Furthermore, referencing of alignment sensor W in relation to first object O1 must take place with the aid of object alignment mark M1. Last but not least, referencing of alignment sensor W in relation to workpiece WS by workpiece alignment mark M2 will be necessary.
In the different machine positions of these calibration steps and also during the subsequent actual machining, scanning unit E of the optical position measuring device scans regions of measuring standard S that have been provided with reference numerals RA, RB, RC and RD in FIG. 1. As illustrated, these regions lie on parts of scanned measuring standard S that are spaced relatively far apart along movement direction x. For example, the spacing of regions RC and RD corresponds roughly to the order of magnitude of the spacing {right arrow over (d)}WB between processing tool B and alignment sensor W inasmuch as the machining of workpiece WS by processing tool B takes place in the vicinity of workpiece alignment mark M2. However, processing tool B is situated at a distance {right arrow over (d)}WB from alignment sensor W, with whose aid workpiece alignment mark M2 is able to be detected in the aforementioned third step. When positioning processing tool B, measuring errors resulting from the deformation of sections of the measuring standard having an approximate length {right arrow over (d)}WB must therefore be expected.
FIG. 2 shows an exemplary illustration of existing long-wave deformations of an employed measuring standard S. Similar deviations furthermore are caused by errors in the machine guidance if measuring standard S is not aligned with processing tool B (Abbé error). FIG. 3 illustrates the position errors or measuring uncertainty Δxrel in the different machine positions according to the afore-discussed procedure when utilizing such a measuring standard S. As illustrated, corresponding measuring uncertainty Δxrel in the environment of relevant position xrel=0 is considerable. xrel denotes the distance of the machining position from workpiece alignment mark M2.